Organic photochromic dyes are widely used in ophthalmic lenses (eyeglass lenses) to provide significant darkening of the lens when exposed to sunlight. When the lenses are returned to indoor lighting conditions, they preferably fade to a colorless state quickly. These properties allow the wearer of the eyeglasses to go from indoor to outdoor lighting conditions without having to change from a “clear” pair of glasses to a pair of sunglasses.
Plastic photochromic lenses have been on the market for well over a decade. Their photochromic performance has improved as the technology has progressed. Measurement parameters reflecting photochromic performance include:                Faded transmission: a high level of visible light transmission through the lens is most desirable (when in room light or away from direct or intense reflected sunlight);        Darkened transmission: a low level of light transmission through the lens is most desirable (when exposed to sunlight);        Dynamic range: the difference in the percent of light transmission through the lens between the fully faded state and the darkened state. A wide range is most desirable;        Thermal stability: sustained low light transmission when the lens while exposed to light in a hot environment (e.g. desert or tropical). Minimizing the reduction of dynamic range at higher temperatures is most desirable;        Time to darken from faded state to darkened state: a short time period is most desirable;        Time to fade from the darkened state to the faded state: a short time period is most desirable; and        The ability to fade in a reasonable amount of time when darkened in a cold environment (e.g. winter weather): a short time period is most desirable.        
These performance characteristics are generally expected to be maintained for at least a two year lifetime.
The time to fade is an important feature of a photochromic lens in today's market. Potential customers are often discouraged by the length of time it takes for the lens to fade completely when moved from bright light (e.g. outdoors) to low light (e.g. indoors) conditions. During this fading period, the lenses go through an intermediate transmission range that many people find unattractive. Defining “fade time” or “time to fade” as the time it takes for a darkened lens to fade through 80% of its dynamic range towards the faded state provides a means of comparing different lens products and formulations. These comparative measurements are made under uniform light and temperature conditions. Using this definition, the majority of current commercial state of the art lenses have a time to fade ranging from 12-30 minutes (at 23° C.).
The time to fade exhibited by a given lens depends on the type of photochromic dye, dye concentration, the type of monomers and other additives used to form the polymer matrix, and the relative concentration of those monomers and additives. The time to darken and time to fade for a dye can be influenced strongly by the rigidity of the matrix containing it. Softer matrices can result in improved photochromic response speeds; however, soft matrices can also lead to reduced temperature stability of the photochromic material. In this case the dye would change color very rapidly, however, the dynamic range of the photochromic would be significantly reduced at higher temperatures (such as those encountered by heating as a result of exposure to sunlight). With most photochromic compounds, heat is the primary driving force for transformation of the dye from the darkened state to the faded state and thus a soft matrix would allow the dye to fade while the object is heated by the sun's radiation. Significant fading while the lens is exposed to sunlight is not acceptable because such fade would allow more ambient sunlight to reach the wearer. The polymer matrix must provide a balance between allowing the dye to change from its colored to uncolored state while retarding a rapid fade in a heated condition. The polymer matrix should also retain its photochromic property such that when the lens is darkened in a cold environment, it may still fade enough to allow the person to see when moving from a sunny to a darker condition such as movement from sunlight to indoors.
In today's market it is desirable to produce lenses with indices of refraction greater than 1.50 for reduced lens thickness and greater cosmetic appeal. The pursuit of higher index often comes at the expense of photochromic performance, as in the case of using styrenic components to increase the index.
One of the early problems encountered when incorporating organic photochromic dyes into plastic materials was obtaining satisfactory photochromic dynamic range and speed from the faded to darkened state and vice versa. In some of the early work done incorporating photochromic dyes into cast polymers, certain monomers were found to promote rapid and intense darkening of the dye in the cast specimen.
Use of the monofunctional lauryl methacrylate is suggested in U.S. Pat. No. 3,565,814, and is one of the earliest examples of a methacrylate specified for improving the darkened state of the formed article. However, this patent did not address the problem of time to fade or temperature dependency.
Maltman et al. (U.S. Pat. No. 4,851,471) suggests using highly reactive monomers to reduce the level of a peroxide initiator that is inherently damaging to the photochromic dye. The dye level is used at relatively high concentration to allow for some loss due to attack during polymerization by the initiator. Examples are given using triethylene glycol dimethacrylate as the primary monomer.
More recently, U.S. Pat. No. 5,914,174 discloses the use of hydrophilic polyethylene glycol based difunctional acrylates and methacrylates (with acrylate being preferred), and monofunctional hydrophobic acrylates and methacrylates (with methacrylate being preferred), with a long chain methylene moiety, for use in composite ophthalmic lenses. The monofunctional component is given as 0-70% of the matrix, with the difunctional monomer at 10% -50%, and an allowance of 0-20% for multifunctional components. The mixture is disclosed for use in a specific approach in manufacturing a composite lens by providing the composition as a thin coating on a cast resin substrate and requires the composition to have a viscosity from about 25 cps to about 150 cps at 25° C. This patent exemplifies that at least 30% of the composition be monofunctional acrylates or methacrylates.
The use of difunctional acrylates and methacrylates in lens materials is described in U.S. Pat. Nos. 6,221,284 B1 by Florent, and 6,329,482 B1 by Henry. These formulations are disclosed as suitable for utilization of photochromic dye dispersed throughout the lens, and are comprised of mixtures of other compounds in the lens composition that provide mechanical strength and high refractive index. The formulation proposed by Florent is based on ethoxylated bisphenol A diacrylate or dimethacrylate with aromatic monovinyl and divinyl components. The patent discusses the use of styrene to provide thermal stability and divinyl benzene to improve the level of darkening in photochromic formulations. Henry discloses the use of other difunctional monomers that increase the index of refraction and promote photochromic performance.
U.S. Pat. No. 5,683,628 offers the use of a family of diacrylates and dimethacrylates based on a chain molecule with a central benzene ring, and specific locations along the chain for incorporation of functional groups to boost the index of refraction, and thus avoid use of styrenics. The resins produced are disclosed as affording excellent heat resistance, mechanical strength, adhesion to a hard coat layer, and moldability.
The casting of photochromic lenses using methacrylates and acrylates was also discussed in U.S. Pat. No. 6,171,525 B1 by Effer et al. The patent requires the use of at least one spirooxazine dye, and calls for a high level of polymerization initiator (at least 1.5% by weight).
While the prior art suggests the use of various combinations of composite monomers, none fully explore and optimize the use of these monomers for control of faded state clarity, darkened state light transmission, time to fade, and temperature stability.
The compositions used in the art thus far also have been only at best partially successful at producing ophthalmic lenses that meet the needs of consumers. Thus, there still exists a need for novel compositions for the preparation of ophthalmic lenses that display high transmission in room light, low transmission in sunlight (thus a wide dynamic range), high thermal stability, and short times for darkening and fading even under cold temperature conditions.